Chromium plating process



United States Patent 3,414,492 CHROMIUM PLATING PROCESS George E. Oleson and Roger M. Woods, Fairfax, Va., assignors to Corillium Corporation, Arlington, Va., a corporation of Virginia No Drawing. Filed Dec. 14, 1965, Ser. No. 523,818 12 Claims. (Cl. 204-51) This invention relates to a process of chromium plating and more particularly to a process for producing a lustrous uniform blue-grey chromium finish on a wide variety of conductive base materials.

An object of this invention is to provide 'a process of electrodepositing an extremely adherent corrosion resistant blue-grey film upon a conductive base which film exhibits excellent wear resistance and which protects the base metal from corrosion. Another object of the invention is to provide a process using conventional chromium plating equipment wherein certain chemical compounds and conditions are provided for obtaining a blue-grey chromium plate. Another object of the invention is to provide a process for finishing decorative trim components' for use in the automotive and appliance fields wherein a blue-grey chromium plate is obtained which has an attractive and unique decorative appearance and also has excellent wearing properties. Other objects and advantages of the invention will become readily apparent from the following detailed description of the invention.

According to the invention, a blue-grey chromium plate is obtained on a conductive base material in a system in which the base metal is cathodic and in which the electrolyte comprises an aqueous solution of chromic anhydrides (CrO containing a chloroacetic acid in the solution. The solution contains from about 110 grams to 450 grams per liter of chromic anhydride and from about 75 grams per liter to about 265 grams per liter of chloroacetic acid. The monochloroacetic acid is preferred, although the diand trichloroacetic acids may suitably be used.

The object to be plated is prepared for plating by conventional plating procedures, which may include degreasing, alkaline cleaning, acid dipping, polishing, sandblasting, etc.

The preferred electroplating bath comprises an aqueous solution containing 210 to 265 grams per liter of CrO and 130 to 165 grams per liter of chloroacetic acid.

The plating bath is prepared in the concentrations as stated above and added to a plating tank to the desired operating level. The tank must be thoroughly cleaned to prevent bath contamination, particularly where sulfate or fluoride type baths have previously been used. Anodes are then placed in the tank and electrolyzed with dummy cathodes for about one hour. The solution is frequently stirred during this period as by air agitation. The bath is then ready for use. The object or part to be plated is then immersed in the bath and is made the cathode of the electroplating system. Anodes of various materials may be employed, but it has been found preferable to use carbon steel anodes over the conventional lead alloy anodes due to the heavy scaling of the lead. Stainless steel and carbon anodes may be employed, but with less satisfactory results. The tank which holds the electrolyte solution is preferably lined with a synthetic resin material such as polyvinyl chloride or other suitable inert material. The plating tank should also be provided with an exhaust system to carry away fumes of the chloroacetic and chromic acids.

The plating bath should be operated at a temperature of about 60l00 F. and preferably at about room temperature, i.e., 70-80" F. The best deposits will usually be obtained at a cathode current density of between 50 and Patented Dec. 3, 1968 600 amperes per square foot, and preferably about 250-350 amperes per square foot.

Plating times will vary according to the thickness desired and the complexity of the object to be plated, but normally the time will vary between about 30 seconds to about ten minutes for decorative applications, and up to several hours for hard chrome applications.

After the object has been plated, it is removed from the bath and rinsed in a conventional manner and dried by air blast or wiped with an absorbent material. The covering power is approximately that of conventional chromium plating, and in cases of extremely deep recesses, internal anodes may have to be used, the same as in conventional chromium baths.

It has been found desirable to maintain the anode area as large as possible with respect to the cathode area in order to maintain the anode current density at a relatively low value to avoid coating the anode with a film, the exact nature of which is not fully understood, but which tends to increase the resistance of the system. The anode area should be as great as and preferably three or four times as great as the cathode area for most efficient operation.

The plating bath may be replenished by the addition of chromic anhydride and chloroacetic acid as they are consumed to bring the concentration of the ingredients in the bath back to the concentration of the ingredients originally present in the bath. A sufiicient accurate control for production use may be achieved by conventional analytical techniques.

The plating bath of the invention which produces a uniform blue-grey chromium finish is applicable to a wide variety of conductive base materials such as copper, nickel, steel, stainless steel, brass and bronze. Other metals such as zinc die cast and aluminum can also be plated after a suitable underplate is applied. For decorative deposits the thickness will be on the order of 5 to 30 millionths of an inch. For hard chrome deposits they will be on the order of over 50 millionths of an inch.

The plate produced by the above-described process is extremely lustrous and blue-grey in color and exhibits an extremely lustrous appearance when plated over a bright or polished surface. Spectral reflectance of the plated article may be reduced by rendering the exterior article matte in appearance prior to the final plating operation. Thus, different etfects can be achieved by varying the surface condition of the base material or underplate. When the surface is a matte or sandblasted finish, the color of the deposit will be a distinctive grey. If the surface is a bright or polished nickel, the deposit will become a more distinctive blue.

It will be appreciated to those skilled in the art that the process involves a chromic acid electrolyte having an extremely high concentration of addition agents compared to the concentration of addition agents to conventional CrO baths with the heretofore unexpected result of obtaining a lustrous blue-grey plating.

Example 1 A conventional polyvinyl chloride lined steel tank containing a carbon steel anode is used in this example and in the following examples. A zinc die cast automobile horn ring was prepared for plating according to conventional procedures as heretofore described. The horn ring was copper and nickel plated according to conventional plating procedures. After rinsing, the horn ring was transferred to the steel tank which contained the blue-grey plating bath which comprised a solution containing 240 grams per liter of chromic anhydride and grams per liter of monochloroacetic acid. The current density was 350 amperes per square foot and the temperature of the bath was 75 F. The plating time was three minutes. The horn ring was then removed and rinsed in cold water, then hot water, and dried. The horn ring has a coating of chrome which had a blue-grey luster.

Example 2 A carbon steel bearing shaft was prepared for plating according to conventional procedures. After rinsing, it was placed in a blue-grey plating bath which contained 210 grams per liter of chromic anhydride and 135 grams per liter of monochloroacetic acid. The current density was 250 amperes per square foot and the temperature of the bath was 90 F. The shaft was plated for a period of one hour. Thereafter, the part was removed and rinsed in cold water, then hot water, and dried. The plated bearing shaft had a hard chrome deposit which had a blue-grey luster.

Example 3 A brass soap dish was prepared for plating as in the above examples and then copper and nickel plated according to conventional procedures. Thereafter, it was rinsed and transferred to a blue-grey plating bath which contained 285 grams per liter of chromic anhydride and 165 grams per liter of monochloroacetic acid. The current density was 400 amperes per square foot and the temperature of the bath was 65 F. The soap dish was plated for a period of five minutes and then removed from the bath and rinsed in cold water, then hot water, and dried. The soap dish had a chrome deposit which had a blue-grey luster.

Example 4 An aluminum flashlight case was prepared for plating according to conventional procedures using the zincate treatment. The case was then copper and nickel plated according to conventional procedures. It was then rinsed and placed in a blue-grey plating bath which contained 165 grams per liter of chromic anhydride and 112 grams. per liter dichloroacetic acid. The current density was 200 amperes per square foot and the temperature of the bath was 70 F. It was plated for a period of three minutes and thereafter removed and rinsed in cold water, then hot water, and dried. The case had a chrome deposit which had a blue-grey luster.

Example 5 A stainless steel building panel was prepared for plating according to conventional procedures as heretofore described. It was then rinsed and placed in a blue-grey plating bath which contained 330 grams per liter of chromic anhydride and 195 grams per liter of trichloroacetic acid. The current density of the bath was 300 amperes per square foot and the temperature of the bath was F. The panel was plated for a period of eight minutes and then removed and rinsed in cold water, then hot water, and dried. The panel had a chrome deposit which had a blue-grey luster.

What is claimed is:

1. A process of producing electrodeposited blue-grey chromium coatings comprising plating an object in an aqueous bath comprising about 110 to about 450 grams per liter of chromic anhydride and about to about 265 grams per liter of chloroacetic acid, using a current density of about 50 to about 600 amperes per square foot and a temperature of about 60 F. to about F., for a period of time sufiicient to give a chromium coating of desired thickness.

2. The process of claim 1 wherein said bath comprises about 210 to about 265 grams per liter of chromic anhydride and about to about grams per liter of chloroacetic acid.

3. The process of claim 1 wherein said chloroacetic acid is monochloroacetic acid.

4. The process of claim 1 wherein said chloroacetic acid is dichloroacetic acid.

5. The process of claim 1 wherein said chloroacetic acid is trichloroacetic acid.

6. The process of claim 1 wherein said current density is about 250 to about 350 amperes per square foot.

7. The process of claim 1 wherein said temperature is about 70 to about 80 F.

8. The process of claim 1 wherein said period of time is about 30 seconds to about ten minutes to obtain decorative blue-grey coatings.

9. The process of claim 1 wherein said period of time is several hours for hard chrome, blue-grey coatings.

10. The process of claim 1 wherein said object to be plated has a previously deposited layer of metal other than chromium.

11. The process of claim 1 wherein the object to be plated is cathode and carbon steel is anode.

12. The process of claim 11 wherein the anode area is at least as large as the cathode area.

References Cited UNITED STATES PATENTS 2,279,830 4/1942 Murray 20451 2,655,471 10/1953 Chester 20451 2,745,801 5/1956 Blaine 20451 3,248,310 4/1966 Schaer 204-51 HOWARD S. WILLIAMS, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

1. A PROCESS OF PRODUCING ELECTRODEPOSITED BLUE-GREY CHROMIUM COATINGS COMPRISING PLATING AN OBJECT IN AN AQUEOUS BATH COMPRISING ABOUT 110 TO ABOUT 450 GRAMS PER LITER OF CHROMIC ANHYDRIDE AND ABOUT 75 TO ABOUT 265 GRAMS PER LITER OF CHLOROACETIC ACID, USING A CURRENT DENSITY OF ABOUT 50 TO ABOUT 600 AMPERES PER SQUARE FOOT AND A TEMPERATURE OF ABOUT 60*F. TO ABOUT 100*F., FOR A PERIOD OF TIME SUFFICIENT TO GIVE A CHROMIUM COATING OF DESIRED THICKNESS. 